The present invention relates to the removal of hydrogen fluoride from gaseous mixtures of hydrogen fluoride and hydrogen chloride. In accord with the present invention, a gaseous mixture containing hydrogen fluoride and hydrogen chloride is placed in contact with, suitably by passing a gas stream through a bed of, an anhydrous alkaline earth metal chloride, such as calcium chloride.
Various techniques to purify hydrogen chloride gas have been proposed which utilize a reaction between hydrogen fluoride and various alkali earth metal halides, other than fluorides, to produce alkaline earth metal fluorides and a purified hydrogen chloride gas. Examples of prior art methods are found in U.S. Pat. Nos. 3,140,916; 3,976,447; and 4,009,214; and British Pat. No. 1,379,793.
In most chemical processes utilizing hydrogen fluoride gas as a fluorinating agent to convert a chlorine-containing organic compound to a fluorine-containing compound, a hydrogen chloride by-product gas is produced containing unreacted hydrogen fluoride. The amount of hydrogen fluoride in the by-product gas varies widely, dependent upon the fluorination reaction, but ranging typically from about 0.5 to about 30.0 percent by weight of the gas mixture. Such by-product gas represents a significant potential economic loss unless the hydrogen fluoride can be separated from the otherwise marketable hydrogen chloride gas. The methods previously proposed either fail to provide a viable separation or fail to reliably and consistently produce a hydrogen chloride product which has a hydrogen fluoride content sufficiently low that it is commercially marketable. If the hydrogen fluoride content is sufficiently low, the hydrogen chloride gas may be liquified and marketed as anhydrous chloride, or it may be adsorbed in water to produce muriatic acid. Muriatic acid is normally sold in concentrations ranging from about 32 to about 36 percent by weight hydrogen chloride. It is desirable that anhydrous hydrogen chloride or muriatic acid products, especially in view of the present ecological concern in regard to low fluoride content of plant effluent, have extremely low concentrations of hydrogen fluoride. It is not unusual at the present time for industrial specifications to recite a maximum of less than 50 ppm (parts per million) residual hydrogen fluoride in anhydrous hydrogen chloride or muriatic acid products.
The present invention is particularly adapted to the treatment of by-product gas mixtures of hydrogen chloride containing hydrogen fluoride in minor amounts as an impurity. By minor amounts is meant that the amount of hydrogen fluoride present is such that it is more economical to recover the hydrogen chloride rather than the hydrogen fluoride. Usually, if the hydrogen fluoride content of the mixture is above about 5.0 percent and, more practically, above about 3.0 percent by weight, it is more economical to initially treat the by-product mixture to recover hydrogen fluoride. Such treatment may suitably be by a low temperature refrigeration process in which hydrogen fluoride is condensed out of the mixture and recovered, or by a reaction with sodium fluoride to produce a bifluoride, followed by a thermal decomposition to recover hydrogen fluoride. Typically, the hydrogen chloride by-product of these processes contains amounts of hydrogen fluoride which render the product non-marketable by today's standards, but such mixtures are aptly suited to use in the present process.
The amount of hydrogen fluoride gas that will be removed by a bed of alkaline earth metal chloride material is dependent on (1) the bed temperature, i.e., the cooler the bed, the more effectively the bed will remove hydrogen fluoride; and (2) the amount of hydrogen fluoride absorbed by the bed, i.e., as the bed progressively becomes converted from an alkaline earth metal chloride to alkaline earth metal fluoride, the less effectively the bed will remove additional hydrogen fluoride.
The reaction of hydrogen fluoride and an alkaline earth metal chloride material, such as calcium chloride, is exothermic. It has now been determined that the temperature increase in such reaction is about 23.degree. C., for each 1 percent by weight hydrogen fluoride. Thus, for a gas mixture containing 2 percent by weight, the calcium chloride bed increases about 46.degree. C., and, for a gas mixture containing 5 percent hydrogen fluoride, the increase in temperature is about 115.degree. C.
The useful life of a calcium chloride bed may be determined by reference to the following equation: ##STR1## Thus, knowing the weight of the initial calcium chloride charge, the hydrogen fluoride content in the gas to be treated, and the rate of gas flow, the useful life of the calcium chloride bed may be calculated.
The present application is directed to a process for removing hydrogen fluoride from gaseous mixtures of hydrogen chloride and hydrogen fluoride in which the initial mixture contains less than about 5.0 percent by weight hydrogen fluoride. The hydrogen chloride product of the present process normally contains less than 20 ppm of hydrogen fluoride.